On 25 April 2015 an earthquake with a magnitude of 7.8 struck Nepal near the historic town of Gorkha. The earthquake caused numerous landslides and triggered avalanches that caused widespread damage, although much less than what would be expected given the magnitude. An analysis of these earthquake- induced geohazards has been published in Science[1] by a unique international research team that includes scientists from Utrecht University and the International Centre for Integrated Mountain Development (ICIMOD).

The earthquake

A shallow (<12 km deep) earthquake and its aftershocks shook central and eastern Nepal and caused over 9,000 fatalities in Nepal, India, China, and Bangladesh. The primary quake also was felt as far as Pakistan and Bhutan. The impacts of the earthquake and its numerous aftershocks, with a total economic cost of roughly USD 5 billion in Nepal alone, are still being felt where families have been shattered and tens of thousands remain in temporary shelters.

The response

In the aftermath of the earthquake, an international team of volunteer scientists, led by Dr Jeffrey Kargel of the University of Arizona and NASA, worked quickly to identify landslides caused by the earthquake. With remote sensing imagery from government and private agencies, rapid assessments of potentially dangerous sites, such as landslide-dammed rivers and glacier lakes, were conducted by the team. “The information collected by team was directly provided to the Government of Nepal by the International Centre for Integrated Mountain Development to assist in relief efforts,” says Dr Arun Shrestha, regional programme manager at ICIMOD.

The landslide inventory

The rapid response resulted in an inventory of 4,312 landslides[2] mapped and analyzed[1] by the interagency team, which includes 64 scientists from around the globe. The total number of landslides was far fewer than those seen in comparable earthquakes, probably due to a lack of surface ruptures and the geology of the region. "We see the largest numbers of landslides in a wide region north of Kathmandu. This region was literally dropped during the earthquake, which may have triggered the landslides. Areas of uplift showed fewer landslides, despite having similar landslide susceptibilities,” explains Dr Kargel. The team also examined satellite evidence for earthquake-induced damage to glacial lakes. With considerable relief to the team and for the people of Nepal and China, the study revealed very few visible effects on the lakes[1].

The Langtang Valley

The Langtang Valley north of Kathmandu was particularly hard hit and over 350 people were killed in a devastating avalanche in Langtang Village and nearby villages in the valley. “We returned to the valley with a research team in October 2015 and the extent of the damage was almost incomprehensible,” says Dr. Walter Immerzeel, Assistant Professor at Utrecht University. These earthquake-induced avalanches at Langtang Village resulted in wind blasts that may have exceeded 300km/h, and the energy released by the falling snow and ice may have had the equivalent energy of 7.6 kilotons of TNT[4], or nearly half the size of the atomic bomb detonated at Hiroshima. “This research represents an important contribution to our understanding of the impacts of the Gorkha Earthquake. It also serves as a testament to the rapid and coordinated response of the global scientific community to the people of Nepal,” says Dr Joseph Shea, glacier hydrologist at ICIMOD.